Concept:The irrigated Nile Basin is famous for large-scale cultivation of a single leading cash crop.
Explanation:The Nile Basin provides fertile soil and irrigation water, which favour commercial agriculture.
Among the listed crops, cotton is the most widely grown cash crop in this region.
Egypt and Sudan, the main Nile Basin countries, have long depended on cotton for export earnings.
Millet and groundnut are mainly food or subsistence crops, while sugarcane is less dominant than cotton.
Therefore, cotton stands out as the most important cash crop in the irrigated Nile Basin.
Answer:A. cotton